Fundamentals of Lightning Protection

Introduction

Lightning is a capricious, random and unpredictable event. Its' physical
characteristics include current levels sometimes in excess of 400 kA,
temperatures to 50,000 degrees F., and speeds approaching one third the
speed of light. Globally, some 2000 on-going thunderstorms cause about
100 lightning strikes to earth each second. USA insurance company information
shows one homeowner's damage claim for every 57 lightning strikes. Data
about commercial, government, and industrial lightning-caused losses is
not available. Annually in the USA lightning causes more than 26,000 fires
with damage to property (NLSI estimates) in excess of $5-6 billion.

The phenomenology of lightning strikes to earth, as presently understood,
follows an approximate behavior:

2. Ground-based objects (fences, trees, blades of
grass, corners of buildings, people, lightning rods, etc., etc.) emit
varying degrees of electric activity during this event. Upward Streamers
are launched from some of these objects. A few tens of meters off the
ground, a "collection zone" is established according to the
intensified local electrical field.

3. Some Leader(s) likely will connect with some Streamer(s).
Then, the "switch" is closed and the current flows. We see
lightning.

Lightning effects can be direct and/or indirect. Direct effects are from
resistive (ohmic) heating, arcing and burning. Indirect effects are more
probable. They include capacitive, inductive and magnetic behavior. Lightning
"prevention" or "protection" (in an absolute sense)
is impossible. A diminution of its consequences, together with incremental
safety improvements, can be obtained by the use of a holistic or systematic
hazard mitigation approach, described below in generic terms.

Lightning Rods

In Franklin's day, lightning rods conducted current away from buildings
to earth. Lightning rods, now known as air terminals, are believed to
send Streamers upward at varying distances and times according to shape,
height and other factors. Different designs of air terminals may be employed
according to different protection requirements. For example, the utility
industry prefers overhead shielding wires for electrical substations.
In some cases, no use whatsoever of air terminals is appropriate (example:
munitions bunkers). Air terminals do not provide for safety to modern
electronics within structures.

Air terminal design may alter Streamer behavior. In equivalent e-fields,
a blunt pointed rod is seen to behave differently than a sharp pointed
rod. Faraday Cage and overhead shield designs produce yet other effects.
Air terminal design and performance is a controversial and unresolved
issue. Commercial claims of the "elimination" of lightning deserve
a skeptical reception. Further research and testing is on-going in order
to understand more fully the behavior of various air terminals.

Downconductors, Bonding and Shielding

Downconductors should be installed in a safe manner through a known route,
outside of the structure. They should not be painted, since this will
increase impedance. Gradual bends (min. eight inch radius) should be adopted
to avoid flashover problems. Building steel may be used in place of downconductors
where practical as a beneficial part of the earth electrode subsystem.

Bonding assures that all metal masses are at the same electrical potential.
All metallic conductors entering structures (AC power, gas and water pipes,
signal lines, HVAC ducting, conduits, railroad tracks, overhead bridge
cranes, etc.) should be integrated electrically to the earth electrode
subsystem. Connector bonding should be thermal, not mechanical. Mechanical
bonds are subject to corrosion and physical damage. Frequent inspection
and ohmic resistance measuring of compression and mechanical connectors
is recommended.

Shielding is an additional line of defense against induced effects. It
prevents the higher frequency electromagnetic noise from interfering with
the desired signal. It is accomplished by isolation of the signal wires
from the source of noise.

Grounding

The grounding system must address low earth impedance as well as low
resistance. A spectral study of lightning's typical impulse reveals both
a high and a low frequency content. The high frequency is associated with
an extremely fast rising "front" on the order of 10 microseconds
to peak current. The lower frequency component resides in the long, high
energy "tail" or follow-on current in the impulse. The grounding
system appears to the lightning impulse as a transmission line where wave
propagation theory applies.

A single point grounding system is achieved when all equipment within
the structure(s) are connected to a master bus bar which in turn is bonded
to the external grounding system at one point only. Earth loops and differential
rise times must be avoided. The grounding system should be designed to
reduce ac impedance and dc resistance. The shape and dimension of the
earth termination system is more important a specific value of the earth
electrode. The use of counterpoise or "crow's foot" radial techniques
can lower impedance as they allow lightning energy to diverge as each
buried conductor shares voltage gradients. Ground rings around structures
are useful. They should be connected to the facility ground. Exothermic
(welded) connectors are recommended in all circumstances.

Cathodic reactance should be considered during the site analysis phase.
Man-made earth additives and backfills are useful in difficult soils circumstances:
they should be considered on a case-by-case basis where lowering grounding
impedances are difficult an/or expensive by traditional means. Regular
physical inspections and testing should be a part of an established preventive
maintenance program.

Transients and Surges

Ordinary fuses and circuit breakers are not capable of dealing with lightning-induced
transients. Lightning protection equipment may shunt current, block energy
from traveling down the wire, filter certain frequencies, clamp voltage
levels, or perform a combination of these tasks. Voltage clamping devices
capable of handling extremely high amperages of the surge, as well as
reducing the extremely fast rising edge (dv/dt and di/dt) of the transient
are recommended. Adopting a fortress defense against surges is prudent:
protect the main panel (AC power) entry; protect all relevant secondary
distribution panels; protect all valuable plug-in devices such as process
control instrumentation, computers, printers, fire alarms, data recording
& SCADA equipment, etc. Further, protect incoming and outgoing data
and signal lines. Protect electric devices which serve the primary asset
such as well heads, remote security alarms, CCTV cameras, high mast lighting,
etc. HVAC vents which penetrate one structure from another should not
be ignored as possible troublesome electrical pathways.

Surge suppressors should be installed with minimum lead lengths to their
respective panels. Under fast rise time conditions, cable inductance becomes
important and high transient voltages can be developed across long leads.

In all instances, use high quality, high speed, self-diagnosing protective
components. Transient limiting devices may use a combination of arc gap
diverters-metal oxide varistor-silicon avalanche diode technologies. Hybrid
devices, using a combination of these technologies, are preferred. Know
your clamping voltage requirements. Confirm that your vendor's products
have been tested to rigid ANSI/IEEE/ISO9000 test standards. Avoid low-priced,
bargain products which proliferate the market (caveat emptor).

Detection

Lightning detectors, available at differing costs and technologies, sometimes
are useful to provide early warning. An interesting application is when
they are used to disconnect from AC line power and to engage standby power,
before the arrival of lightning. Users should beware of over-confidence
in such equipment which is not perfect and does not always acquire all
lightning data.

Education

Lightning safety should be practiced by all people during thunderstorms.
Preparedness includes: get indoors or in a car; avoid water and all metal
objects; get off the high ground; avoid solitary trees; stay off the telephone.
If caught outdoors during nearby lightning, adopt the Lightning Safety
Position (LSP). LSP means staying away from other people, taking off all
metal objects, crouching with feet together, head bowed, and placing hands
on ears to reduce acoustic shock.

Measuring lightning's distance is easy. Use the "Flash/Bang"
(F/B) technique. For every count of five from the time of seeing the lightning
stroke to hearing the associated thunder, lightning is one mile away.
A F/B of 10 = 2 miles; a F/B of 20 = 4 miles, etc. Since the distance
from Strike A to Strike B to Strike C can be as much as 5-8 miles. Be
conservative and suspend activities when you first hear thunder, if possible.
Do not resume outdoor activities until 20 minutes has past from the last
observable thunder or lightning.

Organizations should adopt a Lightning Safety Policy and integrate it
into their overall safety plan.

Testing

Modern diagnostic testing is available to mimic the performance of lightning
conducting devices as well as to indicate the general route of lightning
through structures. This testing typically is low power, 50 watt or less.
It is traceable, but will not trip MOVs, gas tube arrestors, or other
transient protection devices. Knowing the behavior of an event prior to
occurrence is every businessman's earnest hope. With such techniques,
lightning paths can be forecast reliably.

Codes & Standards

The marketplace abounds with exaggerated claims of product perfection.
Frequently referenced codes and installation standards are incomplete,
out dated and promulgated by commercial interests. On the other hand IEC,
IEEE, MIL-STD, FAA, NASA and similar documents are supported by background
engineering, the peer-review process, and are technical in nature.

Summary

It is important that all of the above subjects be considered in a lightning
safety analysis. There is no Utopia in lightning protection. Lightning
may ignore every defense man can conceive. A systematic hazard mitigation
approach to lightning safety is a prudent course of action.

References

API 2003, Protection Against Ignitions Arising out of Static,
Lightning, and Stray Currents, American Petroleum Institute, Washington
DC, December 1991.